Mayfly Swarm Wreaks Havoc in Upper Midwest

Some Wisconsin and Minnesota Residents Have a Big Cleanup Job; Overwhelmed Car Washes

By

Andrea Gallo

July 23, 2014 4:13 a.m. ET

Some Wisconsin and Minnesota residents had a big cleanup job this week after a huge swarm of mayflies emerged from the Mississippi River, blanketing light posts, making roadways slick and lighting up radar screens.

At least one traffic accident was blamed on the infestation, which lasted only a few hours Sunday evening while the bugs conducted their frenzied annual mating ritual.